Anterior cervical discectomy and fusion (ACDF) is the most common surgical procedure to treat degenerative cervical spine disease in the United States. Unfortunately, ACDF does not prevent degeneration of motion segments adjacent to the initial surgery (i.e. adjacent segment disease), and approximately 25% of the patients require reoperation within 10 years of the initial surgery. This study aims to identify factors that may increase the risk for adjacent segment degeneration. This research may help decrease healthcare spending by reducing reoperations and improve patient safety and quality of life. Abstract: The etiology of adjacent segment pathology following cervical fusion remains highly controversial. Adjacent segment disease is believed to result from one or more of the following distinct causes: 1) the natural history of the adjacent disc; 2) disruption of the adjacent segment anatomy due to the initial surgery; and 3) biomechanical stress on the adjacent level following the fusion. The long-term goal of our research is to reduce or prevent symptomatic adjacent segment degeneration in the spine. The overall hypothesis of this study is that adjacent segment kinematics (i.e. translations, rotations, helical axis of motion) and arthrokinematics (i.e disc deformation and facet joint surface interactions) after ACDF are determined primarily by patient-specific anatomy and iatrogenic factors, and not by increased biomechanical stress due to the fusion. A prospective longitudinal study is proposed to determine to what extent patient-specific factors (Specific Aim 1), iatrogenic factors (Specific Aim 2), and altered biomechanics (Specific Aim 3) affect dynamic cervical spine function following fusion. Participants will be C56 (n=22) and C67 (n=22) single-level fusion patients, C456 (n=22) and C567 (n=22) two-level fusion patients, and asymptomatic controls similar in age to the fusion patients (n=22). Patients will be tested prior to surgery, one year post-surgery, and three years post-surgery. At each test, participants will complete clinical questionnaires to assess pain and function, and they will perform full range of motion flexion\extension and axial rotation of the head and cervical spine while biplane radiographs are recorded at 30 images per second. A highly accurate and validated volumetric model- based tracking process and custom data analysis software will be utilized to determine intervertebral kinematics (i.e. translations, rotations, helical axis of motin) and arthrokinematics (i.e. disc deformation and facet joint surface interactions) at each test session. This prospective study will identify the factors that have the greatest effect on adjacent segment mechanics after cervical fusion. If the hypotheses are confirmed, this will provide support for increased attention to patient-specific factors and surgical technique. Alternatively, f the results indicate that adjacent segment mechanics are influenced primarily by increased stress after arthrodesis, this will provide support for increased attention to the design of motion sparing devices.